Fullerenes are broadly defined as the third form of the element carbon after diamond and graphite. Fullerenes are molecular solids that consist of fused six-membered and five-membered rings of carbon atoms. Two general types of fullerenes may be described: Buckyballs and carbon nanotubes. Buckyballs are typified by the essentially spherical C60 molecule. The term Buckyballs also refers to other approximately spherical closed fullerenes such as C70 and higher oligomers. Single wall carbon nanotubes (SWNTs) are elongated, cylindrically shaped members of the fullerene family. Multi-walled carbon nanotubes (MWNTs) are comprised of two or more single walled carbon nanotubes nested within one another.
Fullerenes have come under intense multidisciplinary study because of their unique physical and chemical properties. They have many potential applications across a multitude of product areas ranging from electronics to composites to biotech, medicine and many more. Fullerenes are a desirable allotrope of carbon not only because of their similarity to graphite but also because they have a high surface area that can serve as a storage medium for small molecules. (Hydrogen and lithium are prime examples.)
Advances in a wide range of nanotechnology applications depend critically on the availability of suitable starting materials. In the case of applications and products using carbon nanotubes (CNTs), the critical issues are freedom from defects and attaining low levels of impurities. Both problems are related to growth conditions and parameters. Prior to the discovery of the art described in the present invention it has been virtually impossible to control the defect densities and impurity levels of fullerenes.
The fabrication of fullerenes involves high temperatures and metal catalysts. Two conditions are of critical importance to expanding the use of fullerenes in the future: 1) a simple method of producing them and 2) producing them with as-produced levels of impurity contamination and structural perfection that significantly reduce the extent and cost of post-production processing.
Nanotubes and nanofibers have been produced by several techniques including arc discharge, laser ablation, flame synthesis and a variety of chemical vapor deposition (CVD) methods. Two of the most promising methods for depositing commercial quantities of aligned multiwalled carbon nanotubes are the “floating catalyst” CVD method and the injection CVD method. For the injection method, an organic solvent containing a dissolved organometallic compound that decomposes to form the metal catalyst is injected into a two-zone furnace. Both the solvent and the catalyst vaporize in the first zone, and a carrier gas sweeps the vapors into the second zone where the organometallic compound decomposes to yield nanoparticles of the metal catalyst. Solvent and ligand molecules serve as the carbon source for nanotube growth at the catalyst sites. CNT purity is determined by the extent to which other materials, such as the metal catalyst and various forms of carbon (e.g. amorphous carbon) that may be created during the thermal decomposition and in the growth process, adhere to the CNTs after removal from the growth apparatus. Currently known techniques to remove the unwanted impurities are slow, difficult to use, and costly. There is also little known about ways to reduce the defect density of CNTs once they have been grown.